Heterotrimeric G proteins (G alpha, G beta and G gamma) have pleiotropic roles in plants, but molecular mechanisms underlying them remain to be elucidated. Here we show that Arabidopsis G beta (AGB1) interacts with NPH3, a regulator of phototropism. Yeast two-hybrid assays, in vitro pull-down assays and bimolecular fluorescence complementation assays showed that AGB1 and NPH3 physically interact. NPH3-null mutation (nph3) is known to completely abolish hypocotyl phototropism. Loss-of-function mutants of AGB1 (agb1-1 and agb1-2) showed decreased hypocotyl phototropism, and agb1/mph3 double mutants showed no hypocotyl phototropism. These results suggest that AGB1 is involved in the NPH3-mediated regulation of phototropism. (C) 2014 Elsevier Inc. All rights reserved.